In many applications, it is desirable for a fiber optic cable to include a plurality of optical fibers. With the increased demand for optical communications, there has been a corresponding demand to increase the number of optical fibers, i.e., the fiber count, of a fiber optic cable. By increasing the fiber count of a fiber optic cable, a single fiber optic cable would be able to support additional optical communications channels.
In order to increase the fiber count of fiber optic cables, unitized fiber optic cables have been developed. As shown in FIG. 1, a unitized fiber optic cable 10 includes a number of bundles 12 of optical fibers 14 that are stranded about a common central strength member 16. A unitized fiber optic cable 10 also includes a cable jacket 18 extruded about the bundles 12 of optical fibers 14, and an optional ripcord 22 for facilitating removal of cable jacket 18. As shown in FIG. 1, each bundle 12 of optical fibers 14 includes at least two and, more commonly, six or twelve optical fibers that are stranded together.
Optical fibers 14 are typically tight buffered optical fibers. A tight buffered optical fiber 14 includes a single mode or multi-mode optical fiber that may be surrounded by an interfacial layer. The interfacial layer can be formed of a Teflon® containing material and is surrounded by a tight buffer layer; however, other suitable interfacial layers may be used, for example, an UV acrylate. The tight buffer layer is typically formed of a plastic, such as polyvinyl chloride (PVC). As an alternative to PVC, the tight buffer layer can be formed of a non-halogenated polyolefin, such as a polyethylene or a polypropylene. Still further, the tight buffer layer can be formed of EVA, nylon or polyester.
Each bundle 12 of optical fibers 14 also includes a central strength member 26 about which the plurality of tight buffered optical fibers is stranded. Each bundle 12 of optical fibers 14 further includes a jacket 28 that surrounds the plurality of optical fibers, and an optional ripcord 20 for facilitating removal of jacket 28. Jacket 28 serves to protect optical fibers 14 and to maintain the bundle of optical fibers in a stranded relationship about central strength member 26. Jacket 28 is typically formed of a polymer, such as PVC. As an alternative to PVC, jacket 28 may be formed of a fluoro-plastic, such as polyvinylidene fluoride (PVDF), a fluoro-compound as disclosed by U.S. Pat. No. 4,963,609 or blends of PVC and PVDF or PVC and polyethylene (PE). Jacket 28 is typically relatively thick and, in one embodiment, has a thickness of about 0.8 millimeters.
During fabrication, a bundle 12 of optical fibers 14 is passed through an extruder cross head and jacket 28 is extruded thereabout in order to maintain the optical fibers in position within the bundle. Since the tight buffer layer of the tight buffered optical fibers 14 is typically formed of a plastic, the plastic that is extruded to form jacket 28 will tend to adhere to the tight buffer layer of the tight buffered optical fibers 14 in the absence of a barrier therebetween. In this regard, the plastic that is extruded to form jacket 28 of a bundle 12 of optical fibers 14 may partially melt the outermost portion of the tight buffer layer of the tight buffered optical fibers 14 such that jacket 28 and the tight buffered optical fibers will adhere to one another as the plastic cools. Unfortunately, the adherence of the tight buffered optical fibers 14 to the surrounding jacket 28 generally decreases the performance of the optical fibers. In this regard, signals propagating along optical fibers 14 generally experience greater attenuation as fiber optic cable 10 is bent or flexed in instances in which the tight buffered optical fibers are adhered to jacket 28 since the optical fibers will no longer be free to move relative to jacket 28 in order to accommodate bending or flexure of fiber optic cable 10.
Each bundle 12 of optical fibers 14 therefore also generally includes a barrier 30 disposed between the plurality of tight buffered optical fibers and jacket 28 in order to separate the tight buffered optical fibers from jacket 28 and to prevent adherence therebetween that otherwise would result from the extension of jacket 28 about optical fibers 14. As such, optical fibers 14 can move somewhat relative to jacket 28 as fiber optic cable 10 is flexed. Barrier 30 is typically formed of a layer of strength members, such as aramid yarn, that are typically stranded about the optical fibers. The layer of strength members is also generally relatively thick and may have a thickness of about 0.2 mm in one embodiment.
Each bundle 12 of optical fibers 14 is typically stranded about common central strength member 16 of fiber optic cable 10. Like central strength member 26 of each bundle 12 of optical fibers 14, central strength member 16 of fiber optic cable 10 is typically formed of a relatively stiff fiber or glass reinforced plastic, or a relatively flexible combination of aramid fiber that may or may not be overcoated with a plastic material. Fiber optic cable 10 also includes a protective cable jacket 18 that surrounds each of the bundles 12 of optical fibers 14. Cable jacket 18 is typically formed of a plastic, such as PVC. As an alternative to PVC, cable jacket 18 may be formed of a fluoro-plastic, such as PVDF, a fluoride-compound or blends of PVC and PVDF or PVC and PE.
As described above in conjunction with jacket 28 that surrounds each bundle 12 of optical fibers 14, cable jacket 18 is also typically extruded over the plurality of bundles of optical fibers. As a result of the plastic materials that form cable jacket 18 and the jackets 28 that surround the respective bundles 12 of optical fibers 14, cable jacket 18 and the jackets that surround the respective bundles of optical fibers may also adhere to one another following the extrusion of cable jacket 18 about the bundles of optical fibers. While the adherence of cable jacket 18 to the jackets 28 of the respective bundles 12 of optical fibers 14 does not impair the performance of fiber optic cable 10 as significantly as adherence between jacket 28 of a bundle 12 of optical fibers 14 and the tight buffer layer of the tight buffered optical fibers, the adherence of cable jacket 18 and the jackets of the respective bundles of optical fibers does disadvantageously impair the flexibility of fiber optic cable 10 somewhat.
Accordingly, fiber optic cable 10 can also include a surface coating on at least that portion of the exterior surface of jacket 28 of each bundle 12 of optical fibers 14 that otherwise would be in contact with cable jacket 18. The surface coating is typically formed of powdered talc that serves to prevent or reduce adhesion between cable jacket 18 and the jackets 28 of the respective bundles 12 of optical fibers 14.
Unitized fiber optic cable 10 as depicted in FIG. 1 is generally relatively large. For example, unitized fiber optic cable 10 depicted in FIG. 1 having six bundles 12 of optical fibers 14 stranded about a central strength member 16 with each bundle of optical fibers having six tight buffered optical fibers stranded about a respective strength member 26 generally has a diameter of about 18.8 millimeters. In many applications, it is desirable to minimize the size of fiber optic cable 10 while maintaining or increasing the number of optical fibers 14 within fiber optic cable 10. As such, it would be advantageous to develop a unitized fiber optic cable having a relatively high fiber count while also being somewhat smaller.